1. Field of the Invention
The present invention relates to a method and circuit for correcting a track zero-crossing signal generated from a signal detected by an optical head, in an optical disk system for effecting seek control by a track crossing signal obtained by irradiating an optical beam from the optical head to an optical disk, in order to make the seek control more reliable.
In comparison with a magnetic disk system, an optical disk system and an opto-magnetic disk system have the advantage that memory capacity is greater, but their application is limited because access speed is low.
The major proportion of said access speed is occupied by the seek operation. If this seek can be carried out at high speed and moreover, accurately, the access speed can be improved and the range of the application of the optical disk system and opto-magnetic disk system can be expanded. Therefore, a high speed and accurate seek operation is required.
2. Description of the Related Art
In an optical disk system, an optical head for irradiating an optical beam to an optical disk and a motor (a voice coil motor) for driving the optical head in a radial direction of the optical disk are provided to the optical disk (opto-magnetic disk), which is rotated by a spindle motor.
In the optical disk, on the other hand, a track is disposed between guide grooves and the reflected beam of the optical beam, irradiated from the optical head, from the optical disk, provides a track error signal. A track servo control circuit obtains the track position error of the optical beam from a track error (tracking error) signal TES from the optical head, and controls the optical head so that the optical beam irradiates the track.
This track error signal TES is a sinusoidal wave for each track crossing period and the actual speed of the optical head can be measured from the period of this track error signal, and the number of crossed tracks can be known from the number of the track error signals TES.
A seek control unit acquires the actual speed of the optical head and the present track position from the track error signal TES, effects the seek control of a motor and positions the optical head to a desired track position.
In the optical disk, a mirror portion is disposed at the tail of an ID portion preformatted for each track (address information which is in advance recorded), and the guide grooves are interrupted at this mirror portion. In an optical disk system employing a system that detects tracking offset by this interrupted portion, the mirror portion is used for the detection of tracking offset. Incidentally, the optical disk system disclosed in this specification does not positively use said mirror portion.
Since said mirror portion is disposed, the track error signal TES is broken from the ID portion to its tail part when a seek is carried out at a relatively low speed of the track crossing period of 50 to 100 msec, and excessive pulses appear in a track zero-crossing signal TZC generated by slicing the track error signal TES by a zero level.
A frequency of the track error signal TES is about 500 KHz during the high speed seek. Therefore, the amplitude itself of the track error signal TES becomes small because of the frequency characteristics of an amplifier, and if the ID noise appears on the signal, track crossing cannot be detected by the zero-crossing signal TZC.
To correct this phenomenon, Japanese Unexamined Patent Publication (KOKAI) No. 2-137129, for example, discloses a method that corrects the track zero-crossing signal by providing a correction circuit that generates pseudo pulse signals.
This prior art method measures the period T of the track zero-crossing signal TZC, generates the pseudo pulse when an edge pulse of the track zero-crossing signal TZC does not appear within an interval 1.5 T and adds it to the edge pulse to thereby correct the track crossing pulse.
However, the conventional technique involves the following problems.
(i) Since the pseudo pulse is merely inserted, the excessive track zero-crossing signal TZC resulting from the breakage of the track error signal TES described above cannot be eliminated, and an error occurs in the number of crossed tracks and an accurate seek operation cannot be made.
(ii) Since the pseudo pulse is generated at the position of 1.5 times of the period T described above, the speed greatly deviates from actual speed and the speed control cannot be made correctly when the speed is obtained from the period of the track zero-crossing signal TZC.
(iii) The edge pulses of the track zero-crossing signals TZC within the 1.5 times of the previous period are all handled as correct pulses even though the waveform of the track zero signal TES is previously disturbed. Therefore, the detected speed deviates greatly from the actual speed when the speed is obtained from the period of the track zero-crossing signal TZC, and the speed control cannot be correctly made.
(iv) If the amplitude of the track error signal TES is small as described above, the pseudo pulse cannot be generated because the track zero-crossing signal TZC cannot be continuously obtained. Accordingly, an error occurs in the number of crossed track and an accurate seek operation cannot be made.